The present invention relates to the general field of turbine engines, and it applies in preferred manner to the field of aviation.
The invention relates more particularly to regulating the flow rate of fuel for a turbine engine of an aircraft, such as a turbojet, for example, during a stage of starting the aircraft.
In known manner, the flow rate of fuel for a turbojet is regulated by generating appropriate fuel flow rate commands for the purpose of ensuring that the mass flow rate of fuel injected into the combustion chamber of the turbojet does not exceed a certain (lower or upper) limit beyond which a malfunction of the turbojet may be encountered, such as for example shutdown or surging of a compressor of the turbojet.
Such regulation is conventionally performed in an open-loop using fuel flow rate commands generated on the basis of a relationship, or more precisely on the basis of a network of pre-established relationships, giving the flow rate of fuel to be injected into the combustion chamber for various different reduced (normalized) speed values of a compressor of the turbojet (e.g. a high-pressure compressor for a two-spool turbojet).
In general, two distinct groups of networks of pre-established relationships are taken into consideration:                a first group of relationship networks for ensuring ignition in the combustion chamber, and providing a fuel flow rate command written WMCmd as a function of at least the reduced speed of the compressor which is written XNr, in other words:WFCmd=f(XNr); and        a second group of relationship networks, also known as C/P limits (referring to the ratio of the flow rate C of fuel injected into the combustion chamber divided by the static pressure P measured at the outlet from the combustion chamber), specifying the fuel flow rate for managing the turbojet spin-up stage until it reaches idling speed. In known manner, one such limit may be written in particular in the following form:        
      WF          PS      ⁢                        T          /          288.15                      =      f    ⁡          (              XNr        ,        PT            )      where WF is the fuel flow rate, PS is the static pressure in the combustion chamber, T is the total temperature at the inlet to the high-pressure compressor, XNr is the reduced speed of the high-pressure spool, and PT is the total pressure at the inlet to the fan.
These various relationship networks are drawn up so as to take account of the specific features of the turbojet and also its sensitivity to various parameters, such as, for example: outside temperature, flight domain, etc.
Presently-designed turbojets present ever increasing performance, and their components (compressor, turbine, etc.) are optimized for operating at high speed, to the detriment of low speeds, and in particular during the starting stage.
This leads to modern turbojets being very sensitive to external conditions (e.g. thermal state of the turbojet, outside temperature, accuracy with which fuel is metered, type of fuel injected, outside temperature, aging of the jet, etc.), and it also leads to wide dispersion in behavior between turbojets.
The operability limits of turbojets that are taken into account during open-loop regulation are thus subject to a large degree of variability from one turbojet to another, which is difficult to predict.
Furthermore, the very great sensitivity of such turbojets to numerous parameters makes it laborious, if not impossible, to adjust the above-mentioned command relationships.
It should be observed that for a turbojet having a high-pressure compressor with a compression ratio that is high relative to the number of stages in the compressor, this very great sensitivity also leads to the existence of a relatively narrow corridor between the surging limit and the stagnation limit.
There therefore exists a need for a mechanism for regulating the flow rate of fuel for a turbine engine that is effective and appropriate for the starting stage, which mechanism takes account of the above-mentioned constraints that are imposed by the turbine engines being designed nowadays.